Induction and maintenance of immunological tolerance to transplanted tissue is essential for allograft survival. The main goal of this Program Project is to develop strategies for suppressing immune responses to the engrafted tissue and thereby prolonging graft survival. Allograft acceptance can be substantially improved in model systems by inhibiting naive T cell activation through blockade of costimulatory receptor interactions such as CD28:B7 and CD40: CD40L. Although these results are promising for current transplant therapy, recent data suggest that cross-reactive memory T cells may playa pivotal role in the rejection of allografts. This issue is of considerable significance since previous studies have shown that memory T cells can respond aggressively to antigen in the absence of costimulation, which thereby minimizes the powerful effects of costimulation blockade on graft rejection if the responding cells are derived from memory T cells. Graft rejection often involves both CD4 and CD8 T cells and our recent work has shown that CD4 and CD8 T cells have strikingly different costimulatory requirements. Hence, the goals of this proposal are two fold: (i) to define the mechanisms that regulate commitment of naive CD4 and CD8 T cells to proliferate and differentiate into effector and memory T cells and (ii) to identify differences in the activation programs of naive versus memory T cells. An understanding of these basic processes that regulate the activation and differentiation of naive and memory CD4 and CD8 T cells will allow us to design strategies for selectively blocking anti-allograft responses arising from naive or memory T cells. The specific aims of this project are as follows: 1. To characterize how the duration of antigenic stimulation regulates the activation requirements and commitment of naive CD4 and CD8 T cells to differentiate into 1 degree effector and memory T cells as well as that of memory CD4 and CD8 T cells to redifferentiate into 2 degree effector and memory T cells. 2. To determine the effects of costimulatory signals on the programmed development and activation of naive, central memory and effector memory T cells. 3. To analyze the programmed development of naive and memory T cells at the biochemical level by comparing the activation of key signal transduction pathways and changes in gene expression following antigenic stimulation.